Cardiac myocytes were differentiated in vitro from cultured embryonic stem cells derived from mice. The cultured cells displayed spontaneous beating. Like adult pacemaker cells, the embryonic stem cell derived cells were found to display wave-like spontaneous calcium oscillations which entrain with whole-cell action potentials by means of sodium-calcium exchange. As in sino-atrial node cells, these oscillations were promoted by elevation of intracellular cyclic AMP. This suggests that cells engineered to have constitutively high cAMP levels could form robust pacemakers. We are now embarking on studies of alternative means of creating, from ventricular cells, the coupled calcium and membrane clocks that endow natural sino-atrial node cells with robust beating. In addition to transfection with individual proteins that create the required signalling milieu, we will explore -- in aged tissue - the recapitulation of developmental programs that lead to the SAN phenotype by means of transfection of developmental transcription factors.